Traducción - Latín-Inglés - Ambulat nobiscum, neque tamen ex nostris [est].

which would be the correct translation to> he walks among us but he is not one of us.
Ambulate Inter Nos, Sed Unus Non Est Nostrum
or
Is inter nos ambulat, sed unus nostrum non est
<edit>"Ambulate Inter Nos, Sed Unus Non Est" Nostrum with the correct sentence in Latin that is "Ambulat nobiscum, neque tamen ex nostris [est]."</edit>

The correct sentence in latin would be : "Ambulat nobiscum, neque tamen ex nostris [est]."

You're translation is quite clumsy, because essentially drawn from English (it would give "Go between us, but one isn't ours")

"Ambulat nobiscum, neque tamen ex nostris [est]." litterally tells : "He walks with us, though he isn't part of us, of our group."
You can either put or not "est" at the end of your sentence, it can be implied.

Here you are ;)